1. Field of the Invention
This invention relates to improved data converter circuitry for use in a computerized tomography (CT) or similar radiation imaging data acquisition system.
2. Background Information
Computerized tomography equipment, such as CT scanning apparatus, is widely used as a diagnostic tool for analyzing the internal profile of an object under study, such as for the medical diagnosis of a human body organ. Such equipment can provide a two or three-dimensional composite "picture" of the object by analyzing a plurality of radiation images taken of the object at different orientations. As is usually the case in medical and biomedical diagnostic technologies, improved image resolution and system speed are viewed as desirable objectives. Improved image resolution is directly related to the quality and reliability of medical diagnosis and may permit examination of fine structures previously not capable of being seen by X-ray imaging. Faster system operation tends to reduce the length of time for which a patient must remain essentially motionless, without which clear images cannot presently be obtained. For example, as much as thirty to forty-five minutes may be required with the use of current equipment to collect sufficient data to image just the lower lumbar or spinal region of a patient.
Another important use of CT scanning apparatus is in the field of bomb or explosives detection, such as the detection of explosives located within a suitcase or other piece of luggage destined to be loaded onto an airplane or other passenger vessel. It has been found that thermal neutron analysis (TNA) equipment, once viewed as an attractive solution to the problem of screening "checked" baggage, often provides an excessive number of false detections when attempting to detect small amounts (e.g., less than one pound) of plastic explosive material. In one recently reported study, CT scanning apparatus was demonstrated to more reliably detect such small amounts of concealed explosives. The screening rate (number of bags per hour) has thus far been unacceptable from a practical standpoint, however; a particular need thus exists in this area for faster CT scanning equipment, with higher image resolution.
A usual source of imaging radiation is a source of X-rays located on one side of the object, with the images being developed by one or more X-ray detectors located on the opposite side of the object, whose signal outputs are converted into digital signals which are analyzed by computer. In general, the CT data acquisition system receives the output from the detectors, and, under digital control to correlate a particular signal with a particular orientation, conditions, amplifies and converts the detector output signals into useful digital data form suitable for subsequent analysis.
Typical data acquisition circuitry has four main components: a front-end signal conditioner, an analog signal multiplexer, a data converter, and a digital control. The front-end signal conditioner serves to convert relatively low level analog signals from the detectors, typically photodiodes in state-of-the-art equipment, into low output impedance signals for the rest of the acquisition system. Normally, each detector channel or line has at least some dedicated signal conditioning circuitry associated with it.
The function of the conventional multiplexer is to take the signals from the different detector channels and enable them to be processed (in time-sharing fashion) along common channels, thereby reducing the number of components needed in the follow-on circuitry. It is known to those skilled in the art that the detector channels may be representative of either instantaneous readings or integrated values. In the former case, a track-and-hold capacitor is "read" without being discharged, while in the latter, a sampling capacitor accumulates charge between sample times and is zeroed after each reading.
The analog output of the multiplexer is fed into a data converter to transform the analog signals into corresponding digital signal information appropriately converted to digital form. The whole process operates under the direction of the digital control circuitry.
The data acquisition circuitry of conventional high performance CT systems utilizes more than one data converter because of the combined sampling rate and accuracy requirements. The data converter is often comprised of two primary elements, a floating point amplifier and an analog-to-digital (A/D) converter. To ensure that the input to the A/D converter is always greater than some minimum value, the floating point amplifier operates to provide greater amplification for smaller magnitude input signals, with the amount of amplification given to a particular signal being selected as a function of the magnitude of the input signal.
A prior art approach to data conversion circuitry utilizes a programmable or selectable gain amplifier in which gain is set by changing the feedback path through switching the point of connection to a plurality of resistors connected in series between input and output terminals of the amplifier. Because this approach achieves different gains by varying the feedback resistance of the same amplifier, the settling times are long and different for each gain selection. Also, implementation of an offset drift correction (auto-zero) capability is complicated and cumbersome, as each gain configuration requires a different amount of offset or offset value. Furthermore, gain adjustment is complicated because the same resistors affect more than one gain selection.
A second approach found in the prior art utilizes a plurality of amplifiers in parallel, each configured for a different single gain setting, and means for selecting which one of the amplifier paths will be used to amplify a given signal. This approach is faster than the first approach but requires a different amplifier, with a corresponding different settling time, for each gain setting. Also, because separate amplifier configurations are used, each gain setting will require its own auto-zero setting circuitry for offset voltage correction.
Still a third prior art approach to data conversion circuitry can be seen in Acharya et al. U.S. Pat. No. 4,815,118 for "Data Converter For CT Data Acquisition System." There, programmable gain amplifier circuitry having a switchable resistor attenuator stage is followed by a fixed amplifier stage, with gain being selected by varying the configuration of the resistor attenuator stage. A single settling time independent of gain setting is provided, enabling the simple implementation of an auto-zero feature.
In present state-of-the-art CT scanning equipment, on the order of five hundred individual photodiodes are used to provide relatively high resolution images of the body organ or structure under X-ray examination. Increased image resolution in such equipment is tied to the number of photodiodes or detectors employed; thus, it would be expected that one should simply scale up the number of photodiodes, along with the necessary supporting circuitry, in order to obtain better CT performance. Those skilled in the art recognize, however, that increased image resolution requires increased system linearity and dynamic range. None of the aforementioned prior art data conversion circuitry approaches are readily capable of practical and economical use in CT scanning apparatus which provides, for example, twice the image resolution as compared to currently available equipment.
Accordingly, it is an object of the present invention to provide computerized tomography apparatus having improved image resolution. A more specific object is the provision of data converter circuitry for use in CT scanning equipment having more than double the number of X-ray detectors than in present state-of-the-art equipment.
Another object of the invention is to provide CT scanning equipment having vastly superior performance characteristics with largely conventional, relatively low-cost components.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious to those skilled in the art from the description itself, and further in part will be appreciated by those practicing the invention and using the resulting CT scanning apparatus.